1. Field of the Invention
The present invention generally relates to a printer, such as a printer for printing labels, such as self adhesive labels carried on a backing material. Such a printer can have a spring-loaded print head which can be pressed against a counterpressure roller, wherein an active strand of a printing ribbon and the medium to be printed can pass between the print head and the counterpressure roller. In such printers, the printing ribbon can typically be supplied by, and unwound from a first spool, and can be wound, or taken up by a second spool. In addition, between a drive and the take-up spool, and/or between a drive and a drive roller in contact with the medium to be printed, there can also be a slip clutch which can be located in a wheel with a concentric shaft, to which shaft a torque is transmitted. Such a friction clutch can typically have at least one pair of axially spring-loaded friction discs interposed between the shaft and the wheel.
2. Background Information
On printers, e.g. on thermal transfer printers, it has been found to be necessary to use a slip clutch to limit the torque which is exerted on the spool on which the used ink ribbon (thermal transfer ribbon) is wound up. The slip clutch can thereby prevent an unacceptably high tensile stress on the ribbon. Such an unacceptably high tensile stress could occur, for example, if the spool from which the ribbon is unwound is stopped for any reason, in which case the ribbon can stop moving suddenly. In such a case, one manner for preventing the ribbon from tearing is to use a slip clutch.
To achieve the correct feed, it may also prove necessary to transport the medium being printed, e.g. for printing individual labels, with a separate drive roller instead of, or in addition to the counterpressure roller. Here again, in particular in the case of a malfunction, it can generally be necessary to limit the drive moment exerted by the drive roller, to prevent damage to the drive motor or to other mechanical elements.
Known slip clutches are very common, and, for example, have been configured in the form of multiple-disc clutches. When the torque exceeds a predetermined value which can be defined by the selection of both the friction pair and the force of the spring or springs used, the adhesive friction can be overcome and the clutch can slip. In the extreme case, for example, only the drive shaft continues to rotate, while the wheel remains stationary, or vice-versa.
One such known clutch must be assembled on the shaft piece-by-piece. If the slip clutch in question is very small, and its components are consequently also very small, the assembly becomes a tedious and time-consuming operation. Further, if this shaft with the clutch is to be used in a machine where there is not much space available, the operations of servicing and repairing this clutch, e.g. replacing the friction discs, can be particularly time-consuming and consequently expensive. During these service and repair operations, moreover, it is possible for one of the small friction discs or other pieces to fall into the machine, thereby possibly causing a disruption of operation of the entire machine.